Purification of hydrodealkylation reaction vent hydrogen

ABSTRACT

In a hydrodealkylation process the vent hydrogen stream from the reaction zone is contacted with 5 to 20 mole percent of liquid C7 to C9 aromatics in one or two stages. The mixture is then flashed to yield a liquid containing about 95 percent of the benzene entering the flashing zone.

United States Patent 1151 3,666,826 Newberger et a]. May 30, 1972 [54]PURIFICATION OF 3,213,151 /1965 Sherk ..260/672 R HYDRODEALKYLATIONREACTION 3,260,765 7/1966 Asselin ..260/672 NC 3,287,431 1 1/1966Feigelman.. ....260/672 NC 3,291,849 12/1966 King et a1. ..260/672 NC[72] Inventors: Bernard I. Newberger, Brooklyn; Stanley W. Ehrlich, FarRockaway; Manuel N. Primary gxaminer cunis Davis vergara Rego Park an ofAttorney-Nathaniel Ely and Bruce E. Hosmer [22] Filed: Dec. 24, 1970 57ABSTRACT [21] Appl. No.: 101,321 1 In a hydrodealkylation process thevent hydrogen stream from the reaction zone is contacted with 5 to molepercent of E (51. ..260/672 R, liquid C1 to C9 aromatics in one or twostages The mixture is [58] l 672 NC then flashed to yield a liquidcontaining about 95 percent of the benzene entering the flashing zone-[56] References Cited 7 Cl 1 Drawing Figure UNITED STATES PATENTS3,188,359 6/1965 Lempert et a1 ..260/672 NC 62 Feed 0 65 F I I4 I6 I9Smbilize, Benzene Tower 4O 42 I2 30 39 Feed 1 Hydrogeng Heater Reactor y48 Tower f f Toluene Tower Overhead Stream rL Aromatics BACKGROUND Animportant consideration in a flowsheet for the hydrodealkylation ofalkylated aromatic hydrocarbons is the handling of the vent hydrogenvapor. This vapor contains a substantial amount of benzene. The exactamount is a function of the amount of net effluent gas as well as thetemperatures and pressure of the effluent flash drum. However, thebenzene in this effluent can be in the range from 2-10 percent of thetotal benzene production. It is, therefore, essential to the efficiencyof the overall process that this benzene be recovered.

Although the prior art which used many contacting stages, of which U.S.Pat. No. 3,287,431 is an example, has been able to recover this benzene,it now is proposed to obtain the same results more economically by usingfewer stages.

The aromatics content in the vent hydrogen is about 1 to 1% volumepercent and is comprised mainly of benzene and toluene in ratios between5:1 to 20:1. When the ratio of toluene to benzene in the vent hydrogenis greater than 8:1 then the danger of freezing at purificationtemperatures of as low as 200 F is removed.

In the prior art, the method of recovering aromatics and also to reversethe benzene to toluene ratio has been to use a vent hydrogen absorber.In the absorber, a small amount of toluene and heavier aromatics areused to scrub the vent gas in multi-tray towers. While this method iseffective, it is an expensive method.

SUMMARY Instead of an absorber for reversing the benzene to tolueneratios and recovering the benzene, it is still possible to takeadvantage of the large differences in the equilibrium constants (Kvalues) by using a larger quantity of contacting liquid with only one ortwo stages.

In the operation of hydrodealkylation unit to make benzene ornaphthalene, the net reactor effluent is flashed in a flash drum at atemperature between about 100 to 150 F and at a pressure between about500 to 600 psig to produce a vapor and liquid stream. The purpose ofthis flash is twofold.

It first provides a liquid stream comprised mainly of product aromaticssuch as benzene or naphthalene and unconverted aromatic feed. The liquidstream has dissolved in it a small amount of light gases and saturatedhydrocarbons resulting from the hydrodealkylation reactions, such ashydrogen, methane, ethane, propane, hydrogen sulfide, thiophene, etc.Some of these are removed in the stabilizer; and subsequentfractionation results in pure product.

Secondly, a vapor stream containing about 55 percent hydrogen is formed.In the vapor, the aromatics content is about 1 to 1% volume percent andis comprised mainly of benzene and toluene in a range from 5:1 to 20:1.It is undesirable to vent these aromatics because this represents aneconomic waste of valuable product and feed. The high benzene to tolueneratio is also detrimental if a hydrogen purification unit is used toupgrade the purity of the vent gas.

In order to take advantage of the differences in equilibrium constantsby flashing, an amount of liquid toluene or heavier aromatics equal toabout 5 to 20 mol percent of the vent gas is intimately admixed with thevapor stream. The resulting vapor-liquid equilibrium mixture is cooledbelow ambient conditions by refrigeration and then separated in a drum.The net result is that at least 95 percent of the benzene gets into theliquid phase and only a small amount of toluene is vaporized. Anothermethod of achieving these results is to inject the liquid aromatics intwo consecutive portions, which may not be of equal amounts, while alsousing refrigeration, as above, to perform the flashes at a somewhatlower temperature than ambient in order to decrease the K value ofbenzene.

Single stage mixing is used when a toluene to benzene ratio of less than8:1 is sufficient. In this case the vent hydrogen is not sent to ahydrogen purification unit.

Two stage mixing is used when a toluene to benzene ratio of greater than8:1 is required. In this case the vent hydrogen is sent to a hydrogenpurification unit.

An object of this invention is to minimize the loss of total aromaticsin the vent hydrogen stream.

Another object of this invention is to control the ratio of toluene tobenzene in the vent hydrogen stream without using an absorber.

Further objects of this invention will become apparent from thefollowing description of the preferred embodiment thereof and as moreparticularly shown in the attached drawing.

DESCRIPTION OF THE DRAWING The drawing is a schematic flow diagram of ahydrodealkylation process wherein the vent hydrogen stream enters aliquid vapor equilibrium staging process for purification.

DETAILED DESCRIPTION OF THE DRAWING Hydrogen is passed in line 12 andthrough compressor 18 to line 30 prior to combining with the feed. Thealkylated aromatic hydrocarbon feed passes in line 10 and part of thisfeed mixes with hydrogen feed in line 39 via line 14.

Toluene or mixed alkylated aromatic hydrocarbons are suitable as feedmaterial. Toluene is the most preferred feed while mixed alkylatedaromatics containing considerable toluene is less preferred and mixedalkylated aromatics containing no toluene is least preferred. The abovefeeds, in order to be suitable, must contain less than about 1 percentbenzene but may contain some saturated non-aromatics. Diolefins andolefins contained in the feed must be removed to make a feed suitablefor use in the hydrodealkylation system and it is desirable to minimizethe thiophenic sulfur in the feed stream. Such treatment steps are notshown in this system as the most preferred feed is a toluene feed.

The combined hydrogen and feed in line 39 is mixed with recycle hydrogenin line 83 and rich oil in 28 and is heated at 40 by exchange againstreactor effluent. At this point a stream 65 from the stabilizer 60 isadded and the total reactor feed is heated to about 1,200 F in firedheater 42. It then enters the reactor 44.

The reactor 44 is an internally insulated open chamber containing nocatalytic surfaces. The pressure in this reactor is maintained at about600 psig. The reactor is so sized that the residence time of the totalfeed is in the range of about 20 to 40 seconds. The temperature profilein the reactor is controlled by adding quench gas at appropriate pointsin the reactor such as 52. The temperature profile is so controlled thatit does not exceed 1,350 F at any point within the reactor. It is alsocontrolled so that about 75 percent of the alkylated aromatichydrocarbons in the total feed is converted. At the outlet 55 thereactor effluent is quenched to about 1,200 F by liquid quench 56. Whilequenching in this manner is sufficient to stop the reactions which occurin the reactor 44, it does permit reactions to take place which producecertain hydrocarbon impurities which form azeotropic boiling mixtureswith benzene.

The reactor effluent 46 is then cooled by exchange against reactor feedat 40, by exchange against stabilizer feed at 47, and by water at 48. Itis then separated into liquid and vapor in flash chamber 50. Part of theflashed vapor in 51 is passed through compressor 20 and is used toquench the reactor at 52 and also to supply recycle hydrogen for controlof the hydrogen to aromatics ratio at the inlet to the reactor via line83. The net vapor is taken at 53 and cooled to about F in exchanger 54by exchange against the liquid in 28 from the two stage flash drum 34and the vapor from 34 in line 35. If the hydrogen in line 35 is to bepassed to a hydrogen purification unit it would not pass through heatexchanger 54 but it would go directly to the hydrogen purification unit.

The cooled vent hydrogen stream in 57 combines with the feed from line19 or recycle aromatics in 37 before undergoing further cooling to about65 F in 22. The cooled mixture now passes in 25 to the first stage of 34where it undergoes a flashing to produce a liquid stream containingabout to 20 percent benzene in 31 and a vapor stream which contains 0.2to 0.6 percent aromatics in 21 with a toluene to benzene ratio of about2 to l.

The vapor in 21 combines with feed from 16 or recycle aromatics in 84 topass through line 23 to the second flashing stage at 34. In the secondstage, there is a flashing of the mixture to provide a liquid stream in27 which combines with the first stage liquid to leave in 31. The vaporeffluent from the second stage of 34 in 35 contains about 0.2 to 0.5percent aromatics with a toluene to benzene ratio of about 1 l to 1.

Part of the liquid from flash chamber 50 is taken through line 56 andused to quench the reactor effluent at 55. The net liquid from 50 passesthrough and is reduced in. pressure through valve 59 and is heated at 47by exchange against reactor efiluent and then goes by 58 to thestabilizer 60. Ifrecycle aromatics in 37 and 84 are used as thecontacting liquids and an additional quantity of this material isrequired, then the net liquid at 50, after reduction in pressure, iscombined with rich oil in 32.

Light hydrocarbon gases which are present in the stabilizer feed 58 aretaken overhead of the stabilizer 60 at 62. In addition, a smallsidestream is taken at 65, a few trays from the top, and returned to thereactor 44. This sidestrearn is primarily benzene, but containshydrocarbons impurities, formed most probably after the reactor quench,which form azeotropic boiling mixtures with benzene. If these impuritieswere allowed to remain in the benzene product, its premium quality wouldbe lost. Alternatively, if the azeotropic mixtures were removed,expensive high efficiency fractionation would be required and the yieldof premium quality benzene would be reduced. By retuming theseazeotropic mixtures of benzene and hydrocarbon impurities to reactor 44the impurities are destroyed by hydrocracking and the high yields ofpremium quality benzene are produced.

The bottoms from the stabilizer 60 are taken at 66 as feed to the claytower 68. The operating pressure of the stabilizer 60 is maintained atthat necessary to give a bottoms temperature of about 425 F. Thisenables the bottoms 66 to go directly to the clay tower withoutintermediate heating. The stabilizer is furnished with a reboiler 64.

The clay tower effluent 69 goes directly to the benzene tower 70. Thisis a conventional fractionator operating at essentially atmosphericpressure and reboiled at 76. High purity benzene product is taken as theoverhead product at 72. With feeds other than toluene the bottoms 74 arepreferably sent by line 77 to the recycle tower at 78. For toluene feed,the bottoms containing the recycle aromatics are sent directly back tothe reactor through line 75.

The recycle tower 78 is run at atmospheric pressure. The overhead 79 isessentially rich in alkylated aromatics primarily toluene with recyclediphenyl and water vapor. The overhead in 79 passes through condenser 81and separator 85 with the water removed in 86. A portion of the tolueneand recycle diphenyl is returned to 78 and the remainder is sent throughline 80 to be used as recycle for the system.

In the event that it is necessary to use the recycle aromatics as acontacting liquid, then the following procedure can be followed. Theunconverted toluene, the quantity of which depends on per passconversion, can be returned to the reactor 44 through line 37 and 84.The selection of bypasses 37 and 84 is dictated by the requirements in34. If extra lean oil is desired in 34, bypasses 37 and 84 are used andall of line 80 is chilled and used as contacting liquid. If the demandin 34 is low, the unconverted toluene can be sent directly to thereactor via line 39 and in so doing, bypass 34. The rich oil in 31 issent to the reactor in 28. If the lean oil bypass system is utilized tocreate additional aromatic recycle, then the excess rich oil in 32 iscombined with the net liquid from the reactor effluent flash drum 50 andis sent directly to the stabilizer 60 and then to the benzene tower 70and recycle tower 78 for removal of the light components and benzenethat were recovered in 34.

It will be understood that the foregoing description and disclosure doesnot include all of the customary valves, controls and interconduit heatexchangers, but it will be appreciated that temperatures are readilycontrolled and heat recovered in the well-known manner.

Generally, hydrodealkylation operations are conducted in the temperaturerange of l,O00 to 1,500 F and temperature differences can be kept at 200F or less. The operating pressures are normally in the range of 400 to800 psig with a reaction time within the period of 10 to 60 seconds.There will be at least 2 moles of hydrogen per mole of liquid aromaticfeed.

We claim:

1. A process for the hydrodealkylation of alkylated aromatic hydrocarbonfeed which comprises the steps of:

a. passing the feed and hydrogen to a reaction zone;

b. operating the reaction zone between about 1,000 P and about 1,500 Fand a pressure between about 400 psig and about 800 psig;

c. removing a gas-liquid effluent stream from the reaction zone; coolingsaid effluent;

e. feeding said effluent to a first separation zone where ahydrogen-rich vapor is separated from the liquid containing portion ofthe effluent, the treatment of which will hereinafter be described;

f. passing the liquid effluent to a second separation zone to obtain alight hydrocarbon gaseous effluent and a liquid bottoms;

g. passing the liquid bottoms to a third separation zone to obtain abenzene effluent and a liquid bottoms;

h. contacting the hydrogen from step (e) with heavy hydrocarbons toshift the benzene-toluene equilibrium;

i. feeding the hydrogen and heavy hydrocarbons mixture to a flashingzone;

j. flashing the mixture to obtain a liquid phase of 5 to 15 percentbenzene and a vapor phase of 0.2 to 0.5 percent aromatics with a tolueneto benzene ratio from 2 to l to 15 to k. recovering the benzene asproduct from step (j).

2. The process of claim I wherein a sidestrearn of benzene andazeotropic forming hydrocarbon impurities is removed from the secondseparation zone in step (f) and recycled to the reaction zone in step(a).

3. The process in claim 1 wherein the liquid bottoms from the thirdseparation zone is passed to a fourth separation zone for removal of theheavy aromatics as a bottoms and the overhead from this separation zoneis recycled for use as the heavy hydrocarbons in step (h).

4. The process of claim 1 wherein the feed is used as the heavyhydrocarbon in step (h).

5. The process of claim 1 wherein the hydrogen in step (j) has a tolueneto benzene ratio of less than 8 to 1 6. The process of claim 5 whereinthe hydrogen from step (j) has a toluene to benzene ratio of greaterthan 8 to l and is fed to a hydrogen purification unit.

7. The process of claim 1 wherein the liquid bottoms from the thirdseparation zone is recycled for use as the heavy hydrocarbons in step(h).

2. The process of claim 1 wherein a sidestream of benzene and azeotropicforming hydrocarbon impurities is removed from the second separationzone in step (f) and recycled to the reaction zone in step (a).
 3. Theprocess in claim 1 wherein the liquid bottoms from the third separationzone is passed to a fourth separation zone for removal of the heavyaromatics as a bottoms and the ovErhead from this separation zone isrecycled for use as the heavy hydrocarbons in step (h).
 4. The processof claim 1 wherein the feed is used as the heavy hydrocarbon in step(h).
 5. The process of claim 1 wherein the hydrogen in step (j) has atoluene to benzene ratio of less than 8 to
 1. 6. The process of claim 5wherein the hydrogen from step (j) has a toluene to benzene ratio ofgreater than 8 to 1 and is fed to a hydrogen purification unit.
 7. Theprocess of claim 1 wherein the liquid bottoms from the third separationzone is recycled for use as the heavy hydrocarbons in step (h).